The present invention relates to distribution of satellite signals generally and more particularly to multimode optical fiber links useful in such distribution.
The use of optical fibers in cable television networks is well known in the art. The following patents are believed to be representative of the state of the art: U.S. Pat. Nos. 5,467,212; 5,499,047; 4,891,694; 5,517,232; 5,442,472; 5,394,182; 5,361,091 and 5,345,526. In addition, German Patent Document P4334440.2 describes a system in which TV signals are converted into serial ranges of lower frequencies. These signals are converted into optical signals which are sent to individual user receivers where the optical signals are converted back to electrical signals.
Cable television networks normally operate on AM analog signals in the 5-860 MHz frequency range.
Satellite signals are in the GHz range in one of a number of bands, known as L, C, Ku and X bands. Earth station terminals typically downconvert signals to a range of about 0.95 to 2.2 GHz (L-band) by a low noise block converter forming part of most satellite signal receiving antennas. These signals are difficult to transmit via long coaxial links. Single mode optical fibers are known to be used for point to point connections between satellite transceivers and satellite antennas.
In multi-unit dwellings such as apartment buildings, dormitories, prisons and hotels, conventional shared master antenna television (SMATV) distribution systems for satellite television are typically employed. SMATV systems comprise a headend which demodulates, decodes and modulates at low frequency AM television signals which are then distributed using coaxial cable. SMATV systems have a number of disadvantages. One disadvantage is the number of channels offered is small, typically between 10 and 20. Since the signals are ordinary decoded TV signals, the programming is easily pirated by those not licensed to view the programming. Another disadvantage may be inferior picture quality.
Another approach to distribute satellite television signals uses coaxial cable to distribute an L-band signal which is output from a low noise block converter (LNB). Because of the high attenuation of coaxial cable at 1-2 GHz, these systems require numerous line amplifiers to overcome losses as well as equalizers to cancel frequency tilt distortions. These effects set a practical limit on coaxial distribution systems to about 200 users and about 300 meters.
With digital satellite TV signals, the accumulated noise and distortion in coaxial cable distribution may cause network failure. Each of the network components has a small impedance mismatch. This causes small signal reflections which when summed coherently throughout the network may cause a distorted signal. With digital signals, the receivers may not recover clock pulses. This is a known threshold phenomenon and is very difficult to predict.
The present invention seeks to provide an L-band satellite signal distribution system which is greatly superior in performance as compared with the prior art.
There is thus provided in accordance with a preferred embodiment of the present invention a satellite signal distribution system including a headend which receives satellite signals from a satellite antenna, a plurality of satellite signal receive and distribution units, a plurality of optical fiber links which communicate satellite signals between the headend and the plurality of satellite signal receive and distribution units, and a multiplicity of satellite signal links which communicate satellite signals between each of said plurality of satellite signal receive and distribution units and plural satellite receivers.
In accordance with a preferred embodiment of the present invention, the system also includes a satellite antenna which is coupled to the headend.
Additionally in accordance with a preferred embodiment of the present invention, the system also includes satellite receivers and/or decoders which are coupled to at least some of the multiplicity of satellite signal links.
In accordance with a preferred embodiment of the present invention, the headend, the plurality of satellite signal receive and distribution units, the plurality of optical fiber links and the multiplicity of satellite signal links all carry satellite signals in a 1-2.2 GHz frequency range, whether in digital modulated or FM modulated format.
Further in accordance with a preferred embodiment of the present invention, the satellite receive and distribution units include a diplexer which outputs to a single coaxial cable a combination of the satellite signals and non-satellite television signals, i.e., terrestrial, off-air, CATV, security, interactive return path.
There is also provided in accordance with a preferred embodiment of the present invention a satellite signal distribution system including a headend which includes a fiberoptic transmitter that combines satellite signals from a satellite antenna with non-satellite television signals, a plurality of signal receive and distribution units, a plurality of optical fiber links which communicate the combined satellite and non-satellite signals between the headend and the plurality of signal receive and distribution units, and a multiplicity of signal links which communicate signals between each of the plurality of signal receive and distribution units and a plurality of satellite signal receivers and non-satellite signal receivers.
In accordance with a preferred embodiment of the present invention the system includes a laser, preferably located in the headend, that converts the combined satellite signals and the non-satellite television signals into optical signals. Preferably a photodiode, which may be located in the receive and distribution units, converts the optical signals into electrical signals.
Further in accordance with a preferred embodiment of the present invention a splitter is provided for feeding the electrical satellite signals and the non-satellite television signals to a satellite receiver and a television set or CATV receiver, respectively.
Additionally in accordance with a preferred embodiment of the present invention the system includes a plurality of a pair of splitters in communication with the photodiode, each the pair comprising a splitter for satellite signals and a splitter for non-satellite television signals, wherein a plurality of diplexers combines an output of each the pair of the splitters for feeding the satellite signals and the non-satellite television signals to a satellite receiver and a television set or CATV receiver, respectively.
Still further in accordance with a preferred embodiment of the present invention the system includes a non-satellite television return path for sending a return signal from a user to a source of a non-satellite television signal. The return signal may be sent on separate fibers, or alternatively, the return signal may be sent on a fiber by means of a wavelength division multiplexer.
There is additionally provided in accordance with a preferred embodiment of the present invention an optical fiber link for RF signals comprising an optical transmitter, a multimode optical fiber and an optical receiver.
It is a particular feature of the present invention that RF signals, i.e. analog signals, whether or not digitally modulated, having a frequency in excess of about 1 GHz, are carried on a multimode optical fiber.
In accordance with a preferred embodiment of the present invention, the optical transmitter comprises a vertical cavity surface emitting laser.
Further in accordance with a preferred embodiment of the present invention, the optical transmitter comprises an edge emitting laser.
There is additionally provided in accordance with a preferred embodiment of the present invention a star network comprising a plurality of optical fiber links of the type described above. Preferably, the optical transmitters are placed at the hub of the star.